I never know the rules about Amazon and Steam... like if they activate on Steam or not. It seems some stuff does and some stuff doesn't. I assume since it's EA, they don't?

Amazon has very recently taken great strides in getting DRM information clearly stated on the page of each digital download item. But be warned that if you're buying a bundle, you'll need to check each items individual page for DRM information as their bundles often have mixed DRM.

Is The Sims Medieval stand alone or do I need to buy original one? (if so, which one) Thank you for your help.

It's not a bad game, but it has nowhere near the replay value as The Sims. It's amusing for a few play throughs, but it gets repetitive very quickly. At these prices it's worth checking out. But it's original price? no way.